1. Field of the Invention
The invention relates in general to a phosphor and a light emitting device comprising the same, and particularly a phosphor comprising a composition formula of AaSi5OtNn:EuZ and a light emitting device comprising the same.
2. Description of the Related Art
In recent years, a light emitting device utilizing a semiconductor for emitting light has been intensively used. In particular, a light emitting diode (LED) has been developed successfully. Compared to a conventional light emitting equipment, such as a cold cathode fluorescent lamp and an incandescent lamp, the light emitting device utilizing the light emitting diode has advantages of higher emitting efficiency, smaller volumes, lower power consumption, and lower cost. Therefore, such light emitting device is used as various light sources. A semiconductor light emitting device comprises a semiconductor light emitting element and a phosphor. The phosphor can absorb and convert light emitted from the semiconductor light emitting element. The light emitted directly from the semiconductor light emitting element and light converted and emitted from the phosphor can be mixed for use. The light emitting device can be used in various applications, such as a fluorescent light, a car lighting, a display device, and a backlight for a liquid crystal display. A white light emitting device is used extensively. The current white light emitting device is formed by using YAG phosphor (Y3Al5O12:Ce) having the element cerium as an active center with a semiconductor light emitting element for emitting a blue light. However, the chromaticity coordinates of the mixed lights from the Y3Al5O12:Ce phosphor and the blue-light semiconductor light emitting element is on a junction line between the chromaticity coordinates of the blue-light semiconductor light emitting element and the Y3Al5O12:Ce phosphor. Therefore, the emitted mixed light is a white light lacking a red light, and is insufficient for color rendering property and color saturation property. In addition, the Y3Al5O12:Ce phosphor has a major excitation spectrum area inconsistent with the light emitting region of the semiconductor light emitting element, thus the phosphor generates poor conversion efficiency of the excited light. It is not easy to obtain a white light source for high brightness. For solving the problem of poor color hue and low brightness, it is developed to mix the YAG:Ce phosphor with a phosphor for emitting a red light, and improve quality of the red light phosphor, for increasing luminous brightness.
However, a phosphor absorbing a blue light and converting it into a red light is rare. The materials of development researches are focused on nitride and oxynitride phosphors. The known phosphors comprise Sr2Si5N8:Eu using the element Eu as the active center, CaAlSiN3:Eu, and sialon phosphor having a formula of MzSi12−(m+n)Alm+nOnN16−n:Eu. However, luminous brightness of the Sr2Si5N8:Eu phosphor is poor, and thus an application of the phosphor is limited and not wide. The sialon phosphor has adequate durability, but luminous brightness of which is poor, thus the phosphor is not popular. The CaAlSiN3:Eu phosphor has better durability, and, compared to the sialon phosphor, has better brightness, however, there is still a demand for further increasing the luminous brightness for the phosphor for a light emitting device exhibiting higher light emitting efficiency.